Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Nat. Hazards Earth Syst. Sci. Discuss., 3, 6117–6148, 2015 www.nat-hazards-earth-syst-sci-discuss.net/3/6117/2015/ doi:10.5194/nhessd-3-6117-2015 NHESSD © Author(s) 2015. CC Attribution 3.0 License. 3, 6117–6148, 2015 This discussion paper is/has been under review for the journal Natural Hazards and Earth The unrest of System Sciences (NHESS). Please refer to the corresponding final paper in NHESS if available. S. Miguel volcano (El Salvador, CA) The unrest of S. Miguel volcano A. Bonforte et al. (El Salvador, CA): installation of the monitoring network and observed Title Page volcano-tectonic ground deformation Abstract Introduction Conclusions References 1 2 2 2 2 1 A. Bonforte , D. Hernandez , E. Gutiérrez , L. Handal , C. Polío , S. Rapisarda , Tables Figures and P. Scarlato3 1Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania – Osservatorio Etneo, J I Piazza Roma, 2 95123 Catania, Italy J I 2Ministerio de Medio Ambiente y Recursos Naturales, Kilómetro 5 1/2 Carretera a Santa Tecla, Calle y Colonia Las Mercedes, San Salvador, El Salvador Back Close 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Sismologia e Tettonofisica, Full Screen / Esc Via di Vigna Murata, 605 00143 Roma, Italy Received: 1 August 2015 – Accepted: 13 September 2015 – Published: 9 October 2015 Printer-friendly Version Correspondence to: A. Bonforte ([email protected]) Interactive Discussion Published by Copernicus Publications on behalf of the European Geosciences Union. 6117 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract NHESSD On 29 December 2013, the Chaparrastique volcano in El Salvador, close to the town of S. Miguel, erupted suddenly with explosive force, forming a more than 3, 6117–6148, 2015 9 km high column and projecting ballistic projectiles as far as 3 km away. Pyroclastic 5 Density Currents flowed to the north-northwest side of the volcano, while tephras The unrest of were dispersed northwest and north-northeast. This sudden eruption prompted the S. Miguel volcano local Ministry of Environment to request cooperation with Italian scientists in order to (El Salvador, CA) improve the monitoring of the volcano during this unrest. A joint force made up of an Italian team from the Istituto Nazionale di Geofisica e Vulcanologia and a local A. Bonforte et al. 10 team from the Ministerio de Medio Ambiente y Recursos Naturales was organized to enhance the volcanological, geophysical and geochemical monitoring system to study the evolution of the phenomenon during the crisis. The joint team quickly installed Title Page a multi-parametric mobile network comprising seismic, geodetic and geochemical Abstract Introduction sensors, designed to cover all the volcano flanks from the lowest to the highest possible Conclusions References 15 altitudes, and a thermal camera. To simplify the logistics for a rapid installation and for security reasons, some sensors were co-located into multi-parametric stations. Here, Tables Figures we describe the prompt design and installation of the geodetic monitoring network, the processing and results. The installation of a new ground deformation network can be J I considered an important result by itself, while the detection of some crucial deforming 20 areas is very significant information, useful for dealing with future threats and for further J I studies on this poorly monitored volcano. Back Close Full Screen / Esc 1 Introduction Printer-friendly Version The S. Miguel volcano, also known as Chaparrastique, is a symmetrical stratovolcano, which reaches 2130 ma.s.l. Its summit crater measures 800 m in diameter and 340 m in Interactive Discussion 25 depth. The Pacayal volcano (currently inactive) is located 6 km NW from the S. Miguel edifice (Fig. 1). 6118 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Geographically, it is located in the department of San Miguel, in the eastern part of El Salvador, at coordinates 13.43143◦ N and 88.271468◦ W. The entire edifice belongs NHESSD to the municipalities of San Miguel, Quelepa, Moncagua, Chinameca, San Jorge, San 3, 6117–6148, 2015 Rafael Oriente and El Tránsito. 5 Geologically, it belongs to the quaternary period (probably with an age of about 50 000 years), and is mainly made up of basaltic and andesitic rocks. However, The unrest of the stratigraphy of the volcano is intercalated with plinian acid deposits (dacitic and S. Miguel volcano rhyodacitic) of the Pacayal volcano. Lava flows and mafic scoria emitted through lateral (El Salvador, CA) fissures (Escobar et al., 2004) are evident on the flanks of the volcano; the occurrence A. Bonforte et al. 10 of such lateral eruptions increases the hazard for the numerous people living on the volcano slopes and requiring a more dense monitoring on all around its flanks. Tectonically, the volcano is located in the eastern segment of the Central Graben Title Page of the country, and it is crossed by local faults with a predominant NW–SE direction (see Fig. 1). A huge fault is located to the north, 10 km away from the volcano, with Abstract Introduction 15 a predominant E–W direction, and it defines the forearc segment that interacts directly Conclusions References with the Cocos Plate. This fault is locally called “El Salvador Fault Zone (ESFZ)”. The forearc sliver shows a regional movement relative to the Caribbean plate towards Tables Figures the northwest, with an average speed of about 15 ± 2 mm per year. (Correa-Mora et al., 2009; Alvarado et al., 2011) J I 20 According to the Ministry of Environment and Natural Resources of El Salvador, around 5000 people living on the northwestern flank of the volcano are those most J I exposed to the volcanic hazard. Back Close In the last 1500 years, the S. Miguel volcano has undergone at least 25 small Full Screen / Esc eruptions, being the most active volcano in the Salvadorean volcanic chain. Since 25 1867, there have been at least 15 explosive eruptions through the central crater Printer-friendly Version (MARN, 2014). The Volcanic Explosive Index (VEI) estimated for that period has been set between 1 and 2 (Schiek, 2008). The latest eruption occurred on 29 Interactive Discussion December 2013, when the volcano ejected a moderate volume of ashes and ballistics 6119 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | through the central crater, damaging several crops on the northern flank and prompting the people working there to take flight. NHESSD It is also important to note that the volcanic hazard of S. Miguel is not confined 3, 6117–6148, 2015 to an eruptive episode involving lava and pyroclastic flows, gas emissions, etc., but 5 also the instability of the steep slopes create a suitable scenario to the generate landslides, debris flows or mudslides. The NW, NE, SE, S and SW flanks show vestiges The unrest of of such historical events. At present, the drainage system of the NW sector towards S. Miguel volcano Las Placitas, La Piedra, Lotificación Hercules and Caserío La Cruz are being affected (El Salvador, CA) by this phenomenon that increases the hazard and needs to be monitored especially A. Bonforte et al. 10 during unrest periods. The aim of this paper is to describe the international co-operation to improve the monitoring during the crisis, the design and installation of the multi-parametric network, Title Page the processing of GPS data and the observed ground deformation. This is the very first experience of such complete a monitoring of this volcano; in particular, the geodetic Abstract Introduction 15 network installed is available for future studies and crisis management and the ground Conclusions References deformation analyses here reported provide a useful first insights into defining the deforming areas of the volcano and their possible significance, where attention could Tables Figures be concentrated for monitoring. J I 2 Background J I Back Close 20 Ground deformation at S. Miguel volcano was studied by Schiek (2008), by applying radar interferometry. Earthquake activity was also studied thanks to a network of six Full Screen / Esc broadband stations around the volcano. The management of the seismic network was carried out by the Servicio Nacional de Estudios Territoriales (SNET) of the Printer-friendly Version Ministry of Environment and Natural Resources of El Salvador (MARN). The equipment Interactive Discussion 25 recorded continuously from March 2007 to January 2008. The SAR data were available for the period from February 2007 to January 2008. The study suggested that the main volcanic activity occurs along a fault crossing the crater in a north–south 6120 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | direction, known as the San Miguel Fracture Zone (SMFZ, see Fig. 1). Schiek (2008) measured a long-term succession of ground inflation and subsidence during that NHESSD period, recording a maximum inflation of 6 ± 0.1 cm before mid-October 2007. After 3, 6117–6148, 2015 this period a maximum deflation of 4 ± 0.1 cm was verified until January 2008. The 5 seismic data were used to model the subsurface geometry of SMFZ, determining that during the inflation period there was an excess of pressure of about 0.05 MPa; for the The unrest of deflation, a value of about −0.05 MPa was estimated. S. Miguel volcano (El Salvador, CA) 3 Continuous GPS Station at S. Miguel volcano A. Bonforte et al. During 2007 and 2008, in the framework of a long-term research project by MARN 10 and the University of Wisconsin, Madison on earthquakes and tectonics, continuous Title Page GPS stations were installed close to the volcanoes of Santa Ana, San Salvador and San Miguel. In the particular case of San Miguel, the GPS station was installed on Abstract Introduction 27 June 2007, in the Piedra Azul village on the southwestern flank of the volcano Conclusions References (Fig.